Fast-breeder nuclear reactor

ABSTRACT

Fast-breeder nuclear reactor includes a generally cylindrical reactor core having axial end zone of breeder material and a center zone of fissionable fuel, and means for cooling the zones with fluid coolant, the fissionable fuel zone having end surfaces located in opposite axial directions of the cylindrical core symmetrical to a transverse plane through the core bisecting the fissionable fuel zone, the fissionable fuel zone having a thickness in the axial direction of the core reducing in radial direction toward the axis of the core, the fissionable fuel therein having a uniform degree of enrichment.

States Patent Spenlte [73] Assignee: Siemens Aktiengesellschaft, Berlinand Munich, Germany Hans Spenke, Erlangen, Germany [22] Filed: Apr. 22,1969 [21], App1.No.: 820,038

Related US. Application Data [63] Continuation-impart of Ser. No.696,434, Jan. 8,

[30] Foreign Application Priority Data Jan. 11, 1967 Germany ..S 107,812Apr. 27, 1968 Germany ..P 17 64 235.2

[52] US. Cl ..176/17, 176/40 [51] Int. Cl ..G2lc 1/02 [58] Field ofSearch ..176/17, 18, 40

[56] References Cited UNITED STATES PATENTS 2,975,117 3/1961 Zinn...176/40 3,201,318 8/1965 Dickenson ..176/30 CUULANT l l '1 l 13,212,982 10/1965 3,287,224 1 H1966 Lowenstein 3,335,060 8/1967 Diener..176/18 X OTHER PUBLICATIONS Paper No. 57- AlF- 34, A New Approach to aMaterials Testing Reactor, by Gruber, Oct. 1957, pp. 1- 6.

Astley et a1. ..176/40 Primary Examiner-Carl D. Quarforth AssistantExaminer-Harvey B. Behrend Attorney-Curt M. Avery, Arthur E. Wilfond,Herbert L. Lerner and Daniel .1. Tick [57] ABSTRACT Fast-breeder nuclearreactor includes a generally cylindrical reactor core having axial endzone of breeder material and a center zone of fissionable fuel, andmeans for cooling the zones with fluid coolant, the fissionable fuelzone having end surfaces located in opposite axial directions of thecylindrical core symmetrical to a transverse plane through the corebisecting the fissionable fuel zone, the fissionable fuel zone having athickness in the axial direction of the core reducing in radialdirection toward the axis of the core, the fissionable fuel thereinhaving a uniform degree of enrichment.

10 Claims, 8 Drawing F lgures Patented April 25, 1972 3,658,643

3 Sheets-Sheet 1 ll Kvoid FAST-BREEDER NUCLEAR REACTOR This applicationis a continuation-in-part of my application, Ser. No. 696,434,f1led Jan.8, 1968.

My invention relates to a fast-breeder nuclear reactor and, moreparticularly, to such a reactor having steam or fluid metal cooling ofthe fuel and breeder zones as well as a shape or geometry which efiect areduction in the coolant void coefficients. Such fast-breeder reactorsare of the general ERR-2 type constructed by the Argonne NationalLaboratory.

In fast-breeder reactors there is a fundamental likelihood that apositive reactivity will be produced when there is a loss of coolantfrom the reactor core, which can cause a power excursion with consequentdestruction or damage to the'core or other regions of the reactor. Thisso-called void effect has two main components one of which is based onvariation of the neutron energy spectrum and the other on variation inneutron leakage losses, because a variation in the moderating phenomenaas well as in the absorption of neutrons results from loss of coolant.It consequently follows that this void effect must be kept as small aspossible for reasons of safety. The minimizing of the void effect can beachieved by increasing the neutron leakage losses with the aid ofspecial core geometries such as for example, pancake, annular or modularforms or shapes. In the last-mentioned example of core geometry, thereactor core is composed of individual modules, each of which possessesits own fissionable fuel, breeder material and reflector zones.

A disadvantage of the heretofore and aforementioned constructions forminimizing the void effect is that in all of the proposals to date, thereactor provided with such construction has a relatively poorefficiency. t

It is accordingly an object of my invention to provide a fastbreedernuclear reactor with a core geometry with a local increase in neutronlosses only 'and therewith a decrease in the maximum void effect, whichaffords an essentially improved reactor efiiciency and economy.

With the foregoing and other objects in view I accordingly providefast-breeder nuclear reactor comprising a generally cylindrical reactorcore having axial end zone of breeder material and a center zone offissionable fuel, and means for cooling the zones with fluid coolant,the fissionable fuel zone having end surfaces located in opposite axialdirections of the cylindrical core symmetrical to a transverse planethrough the core bisecting the fissionable fuel zone, the fissionablefuel zone having a thickness inthe axial direction of the core reducingin radial direction toward the axis of the core.

In accordance with a further recent development of my invention, Iprovide such a fast breeder reactor wherein the degree of enrichment ofthe fissionable fuel is uniform in all radially extending regions of thereactor core.

v In accordance with yet another feature of my invention, thefissionable fuel zone of the reactor is constructed in aquasihomogeneous manner of laterally open fuel elements i.e., as viewedin cross section, the fissionable fuel zone is made up of equalconcentrations of nuclear fuel spaced equidistantly from one another.

In accordance with further features of my invention, the axial end zonesof breeder material and the fissionable fuel zones are defined by anassembly of rod-shaped fuel elements having end portions of breedermaterial and a center portion of fissionable material and, moreover, thefuel elements are disposed substantially parallel to and equally spacedfrom one another.

Other features which are considered as characteristic for the inventionare set forth in the claims.

Although the invention is illustrated and described herein as embodiedin fast-breeder nuclear reactor, it is nevertheless not intended to belimited to the details shown since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings, in which:

FIG. 1 is a graph showing the principal reactivity curve of a knowndisc-shaped fast-breeder'reactor subjected to uniform coolant losses;

FIGS. 2 and 3 are diagrammatic views, partly in cross section, throughtwo different embodiments of the reactor constructed in accordance withthe invention;

FIGS. 3a and 3b are diagrammatic partly cross-sectional enlarged viewsof two embodiments of rod-shaped fuel elements used in the reactor ofFIG. 3;

FIG. 4 is a diagrammatic view of a reactor constructed according to myinvention similar to the embodiment of FIG. 3 though having fissionablefuel and breeder material zones of different relative dimensions;

FIG. 5 is a graph, wherein percentage of incremental coolant void effectis plotted against radial distance from the core center, showing theprincipal reactivity curve of the fastbreeder reactor of this invention;and

FIG. 6 is a diagrammatic, partly longitudinal sectional view of severalfuel elements of the invention.

Referring now to the drawings and first particularly to FIG. 1 thereof,there is shown in a plot of incremental coolant void effect (A k againstradial distance R from the center of the core to the marginal edgethereof, that the integration of the void eflect over various void zoneradii provides a maximum value A k max. located between the center ofthe core and the marginal edge thereof, due to the fact that the leakagelosses of the neutrons are small in the core center and becomeincreasingly greater as one approaches the marginal edge of the core.This maximum value is of critical importance for reliable operation of afast-breeder reactor.

My means of the arrangement and relative dimensioning of the reactorcore'and of the fuel elements comprised by the core, in accordance withthe invention of the instant application, the neutron leakage losses areincreased in those regions of the core which contribute to an especiallygreat positive extent to the maximal void efiect.

The increase and local change of the leakage losses is achieved byreducing the axial thickness of the fissionable fuel region in adirection from the marginal edge toward the center of the cylindricalreactor core and, conversely, by increasing, in a direction from themarginal edge of the reactor core toward the center thereof, thethickness of both of the breeder material regions, as measured in theaxial direction of the core. The axial thickness or height, as viewed inFIGS. 2 and 3, of the fuel region may be reduced quantitatively to anextent that the maximal void effect is below the prompt critical value,i.e., below a l S li portion of the delayed neutrons).

In FIG. 2 which shows schematically and partly in cross section anembodiment of a generally cylindrical fast-breeder reactor coreaccording to the invention, the reduction in the axial thickness orheight of the fissionable fuel zone 1 is constant in a direction fromthe marginal or peripheral surface thereof to the center of the nuclearcore. Thus the axial end surfaces of the fuel zone have a substantiallyconical or concave shape. In the corresponding view of the secondembodiment of the invention as shown in FIG. 3, the reduction in thethickness or height of the fuel zone 1, 2 is stepwise in a directionfrom the margin or peripheral surface of the core toward the centerthereof. In both FIGS. 2 and 3, the reactor core is shown as having apair of axially disposed breeder material covers or end zones 3 and agenerally cylindrical sleeve or annular peripheral zone 4 of breedermaterial. Such breeder zones are also known as blanket zones, and thebreeder material can consist of U-238. The division of the reactor intoindividual zones is produced by suitably inserting into the conventionalnon-illustrated reactor frame, rodshaped fuel elements 5 and 5' (FIG. 3)having respectively shorter lengths 1 and longer lengths 2 of thermallyfissionable fuel 1 such as U-235, U-233 or plutonium in some chemicalform and accordingly longer and shorter lengths of breeder or blanketmaterial 3 such as U-238 or thorium, in some chemica] form as shown inFIGS. 3a and 3b. The annular zone 4 is made up of rod-shaped elementsformed of breeder material, generally, without or with very littleamounts of fissionable fuel. Since the individual fuel elements 5, 5 ofthe reactor can contain a large number of relatively thin fuel rods (notshown), practically any constant reduction in the height or thickness inthe axial direction of the fissionable nuclear fuel zone 1 proper (seeFIG. 2) can be achieved. From a practical viewpoint it should however bepreferable to provide a steptype arrangement of the fissionable fuelzones 1 and 2 according to the embodiment of FIG. 3 because, thereby,substitution or replacement of the fuel element within a zone of thesame height or axial thickness of the fissionable fuel is possible andfewer types of fuel rods having varied distribution or lengths ofbreeder and fissionable materials must be provided.

As shown diagrammatically in FIG. 3, coolant, such as steam or liquidmetal, is circulated through suitable passages provided in the core in aconventional manner.

For further illustration of the geometric construction of thefast-breeder reactor of my invention, the data for a power reactor of2500 megawatts thermal are given hereinafter.

The geometric construction with respect to the shape of the fissionablefuel and breeder material zones is readily apparent from thediagrammatic view of the reactor embodiment shown in FIG. 4. Thefissionable fuel zones and the axial and radial breeder material zonesare identified in FIG. 4 by the same corresponding reference numeralsemployed in FIGS. 2 and 3. In the example of FIG. 4, the thickness i.e.,the elevational dimension as shown in FIG. 4, of the fissionable fuelzone 1 was 47 cm and the radial length thereof i.e., the distance fromthe center of the fissionable fuel zone 1 to either end thereof, was 100cm. The thickness i.e., the elevational dimension in FIG. 4, of thegraduated fissionable fuel zone 2 was 94 cm while the outer radiusthereof from the center of the fissionable fuel zone 1 was 158 cm. Thecombined thickness or elevational dimension of the reactor core in FIG.4, i.e., of both the fissionable fuel zones and breeder material zoneswas 139 cm while the radius thereof was 203 cm.

The aforementioned thermal power arises essentially in the fissionablefuel zones 1 and 2, only 100 megawatts thermal being removed from thebreeder zones or blankets 3 and 4. The mean power density for thereactor of my example was 408 kilowatts per liter, the efficiency of theplant 40 percent. The degree of enrichment in both fissionable fuelzones 1 and 2 was the same. The result of the void effect is apparentfrom FIG. 5, when reference is made to FIG. 1 as a comparison therewithwherein there is shown the principal reactivity curve of a knowndisc-shaped fast-breeder reactor subjected to uniform coolant losses.

As in the embodiment of FIG. 3, the fuel elements in the reactor of FIG.4 extend over the entire elevation of the reactor core, the divisionthereof into the individual zones being effected by suitably arrangingfuel rods having correspondingly longer or shorter sections of breedermaterial and/or fissionable fuel content within the respective fuelelements.

As shown in FIG. 6, the fuel elements employed in the fastbreederreactor of my invention have a laterally open construction, the fuelrods of each fuel element being held together in suitably perforated endplates 6 and spacers, there being no enclosing wrapper tubes for thefuel rods extending between the end plates 6. Thus, a uniform grid orperforated plate structure is able to be provided for all of the fuelrods of the various fuel elements over the cross section of the reactor.

It has also been found that the temperature gradient over the crosssection of the fuel element for most of the fuel elements in the reactorcore is considerably smaller than for a normal conventionally knowndisc-shaped cylinder core. By providing a finer graduation in theelevational dimension, a further reduction of these temperaturegradients can be attained.

Due to the uniform enrichment of the fissionable fuel and the uniformpower density distribution, it can be expected that, with the reactorconstruction of my invention, there is little change in the powerdensity distribution with increasing burn up and formation of plutioniumin the breeder material blankets, and rather, a further linearization iseffected. The temperature gradients over the cross section of arespective fuel element is thereby further reduced so that the meancoolant temperatures rise.

As a mathematically calculated check of this reactor construction of myinvention has shown, for a predetermined maximal void effect, there isobtainable an optimum with respect to the fissionable fuel inventory,the breeding ratios, the long-term behavior and altogether therewith thegeneral economy of a fast-breeder reactor. With such a construction asthat of my invention, the demands made on a fast-breeder reactor withregard to economy and safety are reduced to a common denominator.

According to my invention, the neutron leakage losses are increasedsubstantially only in the center of the fissionable zone and not in themarginal regions thereof, such as occurs in the so-called pancake core.

An advantage deriving from my improved invention, in addition to thesimplification of the production of the fuel elements which are providedwith only a single degree of enrichment when pellets are used therein,is that a marked linearization of the power density distribution isproduced within the reactor core. A further consequence thereof is thatthe temperature gradient over a cross section of a fuel element i.e.,the difference in temperature between the sides of the fuel elementsupport structure facing toward and away from the axis of symmetry ofthe reactor core, is greatly reduced and is made uniform over themultiplicity of fuel elements so that stressing and warping of the majorpart of the fuel elements need hardly be feared any more.

It is possible to devise a core geometry that is optimal with regard tothe power distribution for the axial cooling channels. This is close tothe core geometry, however, which represents an optimum with respect tothe void effect. Thus, from the practical view, both of these objectivescan be attained with adequate approximation with one reactor corelayout.

When laterally open fuel elements are employed, the heating stress inthe individual flow paths of the coolant in the various radiallyextending regions of the reactor core is substantially uniform. Theorifices usually required at the coolant inlet when laterally closedfuel elements, especially, are employed, are therefore superfluous whenthese laterally open fuel elements are used. Thus a practically uniformcoolant outlet temperature is consequently produced at the upper end ofthe reactor core whereby efficiency and economy of the power plant isincreased. Furthermore, the use of laterally open fuel elements isadvantageous over the use of laterally closed fuel elements fromnuclear-physical viewpoints of neutron absorption.

I claim:

1. Fast-breeder nuclear reactor comprising a generally cylindricalreactor core having axial end zones of breeder material and a centerzone of fissionable fuel, and means for cooling said zones with fluidcoolant, both said zones being defined by a mutually parallel assemblyof rod-shaped fuel elements having end portions of breeder material anda center portion of fissionable fuel material, said fissionable fuelzone having end surfaces located in opposite axial directions of saidcylindrical core symmetrical to a transverse plane through said corebisecting said fissionable fuel zone, said fissionable fuel zone havinga thickness in the axial direction of said core reducing in radialdirection toward the axis of said core and having a uniform density offissionable fuel.

2. Reactor according to claim 1, wherein said reactor core includes anannular peripheral zone of breeder material surrounding said fissionablefuel zone.

3. Reactor according to claim 2, wherein said thickness of saidfissionable fuel zone reduces substantially constantly in radiallyinward direction.

4. Reactor according to claim 2, wherein said thickness of saidfissionable fuel zone reduces step-wise in radially inward direction.

5. Reactor according to claim 2, wherein said peripheral zone is definedby an assembly of rod-shaped elements of breeder material.

6. Reactor according to claim 1 wherein the fissionable fuel has auniform degree of enrichment.

7. Reactor according to claim 2 wherein said fuel elements have thebreeder material and the tissionable fuel material

2. Reactor according to claim 1, wherein said reactor core includes anannular peripheral zone of breeder material surrounding said fissionablefuel zone.
 3. Reactor according to claim 2, wherein said thickness ofsaid fissionable fuel zone reduces substantially constantly in radiallyinward direction.
 4. Reactor according to claim 2, wherein saidthickness of said fissionable fuel zone reduces step-wise in radiallyinward direction.
 5. Reactor according to claim 2, wherein saidperipheral zone is defined by an assembly of rod-shaped elements ofbreeder material.
 6. Reactor according to claim 1 wherein thefissionable fuel has a uniform degree of enrichment.
 7. Reactoraccording to claim 2 wherein said fuel elements have the breedermaterial and the fissionable fuel material thereif directly exposed tosaid fluid coolant.
 8. Reactor according to claim 7 wherein said fuelelements are equally spaced from one another.
 9. Reactor according toclaim 8 wherein said fuel elements are formed of parallel fuel rodssecured to one another by spacers at the ends thereof.
 10. Reactoraccording to claim 9 wherein the fuel rods of said fuel elements aresubstantially equally spaced from one another throughout said reactorcore.